The present invention relates to improved silver-glass paste compositions for use in attaching semiconductive elements, e.g., silicon dies, to appropriate substrates.
Prior patents directed to this type of paste include U.S. Pat. Nos. 3,497,774; 4,401,767; 4,436,785; 4,459,166; 4,636,254 and 4,761,224. Typically, these pastes are used for attaching silicon dies to ceramic substrates.
The aforementioned pastes generally comprise the following essential components in the ranges indicated:
______________________________________ Component % by Weight ______________________________________ silver flake 55-75 lead borate glass frit 10-25 resin 0.5-2% organic vehicle 5-20% ______________________________________
Other additives, e.g., silver oxide, thixotropic agents, or the like may also be included.
In a representative die-attachment process, the paste comprising silver flake, glass frit, resin and vehicle is placed in a cavity in a ceramic substrate, the die is placed on the paste and the resulting die/substrate package is fed on a belt onto and through a furnace where the package is heated to remove the organic vehicle and sinter the residual silver and glass to bond the die firmly to the substrate. The final bond layer must be completely free of voids and, as a consequence, the process usually requires a preliminary drying step where vehicle is evaporated followed by firing in the furnace to remove residual vehicle and melt the glass.
The preliminary drying step of necessity is quite lengthy, requiring between about 2-10 hours at 60-80.degree. C., depending on, for example, the die size. Additionally, the ramp rate, i.e., the rate at which the package is fed from the drying step into the furnace, is carefully controlled so as to ensure that organic burnout is at least essentially completed before the sintering of the silver-glass mix takes place. Relatively low temperature (e.g., up to 50.degree. C./minute) ramp rates are very commonly used to insure vehicle removal and optimum results. Belt-type furnaces are normally employed for the firing stage and, depending on the number of temperature zones involved, the dwell time in the furnace can vary from 30-90 minutes or more. Overall, therefore, from preliminary drying through firing, the processing time for effectively attaching a silicon die to a ceramic substrate is a relatively lengthy one.